Detection of micro-structures such as biological or chemical molecules by using super-paramagnetic beads and particles as the labeling component and by using magneto-resistive sensors for detection of such labels is regarded as a promising technique to achieve accurate molecule counting with a resolution of several molecules or even a single molecule. It has the potential to enable fast, efficient, and economical biological applications, such as in-field virus and bacteria detection.
Binding assays to detect target molecules is a widely used technique in the biological, biochemical, and medical communities. The selective bindings commonly used include polynucleic acid bindings or hybridizations involving RNA and DNA, many types of ligand-to-receptor bindings, as well as antibody to antigen bindings. The target molecules in these bindings, for example, proteins or DNA, can also be a distinctive component or product of viruses, bacteria and cells, which may be the actual objects of interest for the detection.
In a binding assay, the binding molecules are attached to a solid substrate as “capture molecules”. When the assay is exposed to a liquid-form sample, where the target molecules attached to a physical label are contained, the binding molecules capture the target molecules with the selective bindings and immobilize the target molecules on the surface. This capture process is called “recognition”. The recognition events can be made to generate detectable signals from the attached labels and, consequently, the presence or absence of a target molecule can be detected.
In various prior art techniques utilizing the labeled binding process, the labels originally attached to the target molecule are also immobilized on the surface after the recognition process. The labels are either bound together with the target molecules on the surface (“sandwich” assay) or are by themselves (“competitive” assay). After the removal of non-bound labels, the bound labels can then be made to generate measurable signals.
When using magnetic beads or particles as the labeling component and an MR sensor as the signal generator, the MR devices are embedded underneath the binding surface and covered by a protective layer. When the magnetic beads or particles are bound to the surface on the top of a MR sensor, they can generate a magnetic field spontaneously, or, for super-paramagnetic beads or particles, in the presence of an applied magnetic field. This magnetic field from the beads or particles can be sensed by the MR sensor, which can then provide a voltage signal when driven by a sense current.
The magnetic labels reported in previous studies [1-10] or as patents [11-13] are usually super-paramagnetic beads or nano-particles that do not possess magnetization at room temperature without externally applied magnetic, field because of the super-paramagnetic effect. Such labels in biological applications are desired because they do not agglomerate at zero field condition. The particles or beads used in this prior art work usually ranges from tens of nanometers to several microns. When the labels are attached to the surface after the recognition process, either multiple labels are attached on one MR device or there is one label per device. However, the sensing mechanism is generally the same for all the previous designs. When the magnetic labels are attached to the MR sensor top surface, the field generated by the magnetic moment of the beads or particles will either act directly on a MR sensor underneath it or will cancel out a portion of the applied magnet field acting on the sensor. For sensors having no labels attached, the magnetic field from the magnetic labels is not present.
FIG. 1 shows a schematic of the scheme described above. The magnetic bead 1 (by ‘magnetic beads’, we include, as well, particles and nano-particles) is attached to the MR sensor surface by the binding pairs 5 after the recognition process. The MR sensor used or referred to is usually a giant-magneto-resistive (GMR) or a tunneling-magneto-resistive (TMR) sensor, which contains a magnetic free layer 2, a non-magnetic spacer layer 3 and a magnetic reference layer 4. Spacer 3 is usually a conductive layer for GMR sensors and an insulator for TMR sensors. Magnetization of the reference layer 4, as represented by Mreference, is fixed in the X axis direction by an exchange field from other magnetic layers underneath, which are not shown in the figure.
Reference layer magnetization does not change direction under normal magnetic fields. In a conventional MR sensor, a bias DC field Hbias is usually applied in the Y axis direction by two hard magnets on the sides of the sensor, so that the free layer's magnetization will be in the Y axis direction when no magnetic field is applied. However, this free layer alignment with the Y axis can also be achieved by making the sensor dimension along the Y axis longer than in X axis due to the shape anisotropy. Because of the shape anisotropy of thin film, the magnetization of the free layer 2, as represented by Mfree, can only rotate freely in the XY plane when a transverse field is applied along the X axis direction and is very difficult to be made to rotate outside the XY plane towards the Z axis.
With a magnetic field applied in the X axis direction, the free layer magnetization rotates away from the Y axis and the resistance of the entire MR junction will change according to R=R0−ΔR cos(θ)/2, where R0 is the base resistance of the sensor, ΔR is the full range resistance change of the sensor and θ is the angle between the magnetization of the reference layer and the free layer. With a DC current applied to the device, where the current can either flow in the XY plane or perpendicularly through the device, the voltage across the device will change because of the resistance change to produce a measurable voltage signal.
In prior studies and patents, several detection schemes were used. One commonly used scheme is applying a magnetic field in the transverse direction [4-10, 12-13], i.e. along the X axis in FIG. 1. Super-paramagnetic beads are also used. When a super-paramagnetic bead is bound to the top surface of the MR sensor, this applied field can magnetize the bead magnetization along the field direction. The bead magnetic moment in turn will produce a magnetic field in the MR sensor below and will partially cancel the originally applied magnetic field acting on the MR sensor. Therefore, the voltage across the sensor when a bead is present is different from when there is no bead attached, under the same applied field conditions, and the presence of the bead is detected by this voltage amplitude difference. A reference sensor to which beads will not attach at any time is normally used for comparison of this voltage difference. During the detection, the applied field can also be a modulated AC field that will induce the same frequency AC voltage across the sensor. By utilizing a lock-in technique, the signal to noise ratio can be enhanced.
Another bead sensing scheme, also known as BARC [1-3,11], is applying a DC field perpendicular to the film plane, i.e. along the Z axis direction in FIG. 1. This DC field serves to magnetize the super-paramagnetic bead moment vertically. The in plane component of the field generated by the vertical bead moment will rotate the free layer magnetizations in the upper. XY plane and low XY plane towards or away from the Y axis at the same time. If the reference layer magnetization is aligned along the Y axis, or a multi-layer MR structure is used, this rotation will produce a resistance change. It is also called “scissoring mode” [11]. This scheme also needs a reference sensor for the detection.
One problem associated with these previously published or patented bead-MR sensor detection methods is that the signal produced by the MR sensor from the magnetic moment of the magnetic labels is relatively small, i.e. less than 10% of the full MR range of the sensor in most cases. This small signal amplitude is especially problematic if high speed detection by a large number of sensors is required as in an applicable binding assay. Since higher speed needs wider detection bandwidth, which in turn introduces additional noise, with the noise power increasing with bandwidth, the low amplitude of the signal becomes a limiting factor. An FMR detection scheme that was recently disclosed in application Ser. No. 11,528.878, filed on Sep. 28, 2006, can help solve the problem in a different way. In general, then, higher detectable magnetic moments from magnetic labels are always beneficial from a signal-to-noise point of view in both conventional and future MR detection methods.